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In December 2005, Republicans were poised to open the Arctic National Wildlife Refuge to oil and gas drilling, an achievement they had sought for decades. Senate Appropriations Committee Chairman Ted Stevens (R-Alaska) had attached the provision to a must-pass defense spending bill and threatened to keep lawmakers in Washington until Christmas if they tried to strip it. Desperate to remove the provision, leaders from national environmental groups turned to a handful of key GOP senators for help.

With only days left before the critical vote, League of Conservation Voters President Gene Karpinski and Defenders of Wildlife Action Fund President Rodger Schlickheisen obtained a private audience with Sen. John McCain (R-Ariz.). McCain had been on both sides of the Arctic drilling issue over the course of his career, and the two leaders of the fight against opening the refuge were eager to know whether he would come down in their column.

His answer disappointed them. In the brief meeting, the senator said he was unwilling to risk blocking a bill involving the military at a time of war -- even though it was clear the broader funding bill would pass quickly and by a wide margin if opponents managed to strip the ANWR provision from it. "We told him, 'This may be the key vote, this may be the time we win this,' " Schlickheisen recalled in an interview. "He said, 'Not on this bill.' That was it."

Ultimately environmental activists were able to defeat the measure with the aid of two Republican senators -- Lincoln Chafee (R.I.) and Mike DeWine (Ohio). But they have not forgotten McCain's decision, and many say it exemplifies his approach to environmental issues.

"There's no question that among a lot of bad Republican votes in the Senate, he's one of the better ones," Schlickheisen said. "He is perhaps the most unpredictable, erratic, of those votes."

McCain has made the environment one of the key elements of his presidential bid. He speaks passionately about the issue of climate change on the campaign trail, and he plans to outline his vision for combating global warming in a major speech today in Portland, Ore.

"I'm proud of my record on the environment," he said at a news conference Friday at the Liberty Science Center in Jersey City. "As president, I will dedicate myself to addressing the issue of climate change globally."

But an examination of McCain's voting record shows an inconsistent approach to the environment: He champions some "green" causes while casting sometimes contradictory votes on others.

The senator from Arizona has been resolute in his quest to impose a federal limit on greenhouse gas emissions, even when it means challenging his own party. But he has also cast votes against tightening fuel-efficiency standards and resisted requiring public utilities to offer a specific amount of electricity from renewable sources. He has worked to protect public lands in his home state, winning a 2001 award from the National Parks Conservation Association for helping give the National Park Service some say over air tours around the Grand Canyon, work that prompts former interior secretary and Arizona governor Bruce Babbitt to call him "a great friend of the canyon." But he has also pushed to set aside Endangered Species Act protections when they conflict with other priorities, such as the construction of a University of Arizona observatory on Mount Graham.

Doug Holtz-Eakin, McCain's senior policy adviser, said the senator does not always please "environmental groups who are single-issue, litmus test" organizations. Instead, he said, McCain seeks to weigh the costs and benefits of each environmental issue.

"Look, he always balances what are the environmental implications of these enterprises and what are the economic benefits that could come from them," Holtz-Eakin said. "That is, in general, an approach which may be harder to read than a flat ideological X or Y, but it's how he reads these things, it's how he evaluates these kinds of decisions."

As a result, McCain scores significantly lower than his Democratic rivals for the presidency, Sens. Barack Obama (Ill.) and Hillary Rodham Clinton (N.Y.), in interest groups' studies of his environmental voting record. McCain's lifetime League of Conservation Voters score is 24 percent, compared with 86 for Obama and 86 for Clinton; Defenders of Wildlife Action Fund's conservation report card gave him 38 percent in the 108th Congress and 40 in the 109th. (McCain has missed every major environmental vote this Congress, giving him a zero rating.)